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Selection of lubricating oil

Selection of lubricating oil

Selecting oil is primarily based on the viscosity required to provide adequate lubrication for the bearing at the bearing operating temperature. The viscosity of oil is temperature dependent, becoming lower as the temperature rises. The viscosity-temperature relationship of an oil is characterized by the viscosity index VI. For rolling bearing lubrication, oils having a high viscosity index (little change with temperature) of at least 95 are recommended.

In order for a sufficiently thick oil film to be formed in the contact area between rolling elements and raceways, the oil must retain a minimum viscosity at the operating temperature. The minimum kinematic viscosity V1 required at the operating temperature to provide adequate lubrication can be determined from diagram 1 provided a mineral oil is used. When the operating temperature is known from experience or can otherwise be determined, the corresponding viscosity at the internationally standardized reference temperature of 40 C, i.e. the oil ISO VG viscosity class, can be obtained from diagram 2, which is compiled for a viscosity index of 95.

Diagram 1. Estimation of the minimum kinematic viscosity at operating temperature

Diagram 2. Conversion to kinematic viscosity at reference temperature (ISO VG classification)

Certain bearing types, e.g. spherical roller bearings, toroidal roller bearings, tapered roller bearings, and spherical roller thrust bearings, normally have a higher operating temperature than other bearing types, e.g. deep groove ball bearings and cylindrical roller bearings, under comparable operating conditions.

When selecting the oil the following aspects should be considered:

  • Bearing life may be extended by selecting an oil where the kinematic viscosity V at the operating temperature is higher than the viscosity V1 obtained from diagram 1. A V > V1 can be obtained by choosing a mineral oil of higher ISO VG viscosity class or by taking an oil with higher viscosity index VI whereby this oil should have at least the same pressure-viscosity coefficient. Since increased viscosity raises the bearing operating temperature there is frequently a practical limit to the lubrication improvement that can be obtained by this means.
  • When the viscosity ratio K = V/V1 is less than 1, an oil containing EP additives is recommended and if K is less than 0.4 an oil with such additives must be used. An oil with EP additives may also enhance operational reliability in cases where K is greater than 1 and medium and large-size roller bearings are concerned. It should be remembered that some EP additives may give adverse effects, see section Load carrying ability, EP and AW additives.
  • For exceptionally low or high speeds, for critical loading conditions, or for unusual lubricating conditions please consult the SKF application engineering service.

Viscosity calculations can be made with the Viscosity calculation.

Example

A bearing having a bore diameter d = 340 mm and outside diameter D = 420 mm is required to operate at a speed n = 500 r/min. Therefore dm = 0.5 (d + D) = 380 mm. From diagram 1, the minimum kinematic viscosity V1 required to give adequate lubrication at the operating temperature is 10 mm/s. From diagram 2, assuming that the operating temperature of the bearing is 70 C, it is found that a lubricating oil of ISO VG 32 viscosity class, i.e. a kinematic viscosity V of at least 32 mm2/s at the reference temperature of 40 C will be required.